/Engine/Source/Runtime/Experimental/ChaosVehicles/ChaosVehiclesCore/Public/EngineSystem.h
When the wheels are in contact with the ground and clutch engaged then the load on the engine from the wheels determines the engine speed. With no clutch simulation just setting the engine RPM directly to match the wheel speed.
void SetEngineRPM ( bool FreeRunningIn, float InEngineRPM )